The present invention relates generally to charging and discharging electrochemical systems and more particularly to improved methods and apparatus for monitoring, manufacturing, charging, and discharging electrochemical cells and systems commonly referred to as batteries.
Although rechargeable batteries are available in many different styles and having many different chemistries, they all share a common dependence upon optimal charging for optimal performance. The consequences of non-optimal charging are largely two-fold.
The first consequence is the most obvious in that failure to provide a full charge to a battery will result in less than the maximum potential energy available to operate the system to which the battery is attached. This results in more frequent charging and less operating time between charges for the powered system. A second consequence is not as obvious, but may become far more important to sophisticated users. This consequence is the loss of battery life which translates directly into cost-of-ownership of the battery. Depending upon the chemistry or construction of a battery, undercharging or overcharging a battery has different but detrimental effects on the usable life of the battery.
The bulk of battery chargers on the market are either constant current chargers that charge to a specified voltage end point, or constant voltage chargers. Some are two stage chargers that charge at a constant current to some voltage and then switch to constant voltage. Constant current chargers are fast but tend to undercharge batteries. Constant voltage chargers are slow. Both types and their hybrid suffer from the fact that their end voltage is set by design and, therefore, is almost never optimum. Some chargers have added sensors to compensate for the temperature coefficient of battery terminal voltage which addresses only one variable. Some newer chargers have implemented dV/dt detection as a method of terminating charging, which still requires the choice of a dV/dt limit, which is also sensitive to various parameters.
Pressure monitoring has also been proposed as a specific solution for lead acid batteries. Some recent chargers use the exothermic or endothermic behavior of some chemistries at full charge as a method to stop charging. Taylor (U.S. Pat. No. 3,816,807) proposes a method of imposing an AC voltage on top of a DC charging current and monitoring the behavior of the complex impedance that results from the AC voltage with a phase detector. He describes a relationship between the real (in phase) impedance and the imaginary (out of phase) impedance that can be used to maintain the output of a charger close to the acceptance limit of a battery. Cherng (U.S. Pat. No. 5,469,043) proposes a method of sweeping a charging voltage from low to high and using information contained in a dV/dI curve to determine the optimum charging voltage. Both of these methods are complex and expensive to implement.